Low temperature transport and storage of amine gas treatment solutions

ABSTRACT

A liquid aminoether acid gas absorbent which is subject to freezing in a cold climatic zone though which the aminoether is to be shipped is rendered freeze-resistant by mixing the aminoether with water prior to transport through the cold climatic zone; the aminoether/water mixture typically contains 10 to 40 weight percent water, based on the weight of the aminoether. The aminoether/water mixture can also be stored in the cold climatic zone without being externally maintained at a temperature above the inherent freezing point of the aminoether.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit under 35 USC 120 from U.S.patent application Ser. No. 61/610,608, filed 14 Mar. 2012.

FIELD OF THE INVENTION

The present invention relates to the absorption of acidic gases from amixed gas streams containing acidic and non-acidic components.

BACKGROUND OF THE INVENTION

The treatment of gases and liquids containing acidic gases such as CO₂,H₂S, CS₂, HCN, COS and sulfur derivatives of C₁ to C₄ hydrocarbons withamine solutions to remove these acidic gases is well established. Theamine usually contacts the acidic gases and the liquids as an aqueoussolution containing the amine in an absorber tower with the aqueousamine solution passing in countercurrent to the acidic fluid. In typicalcases using common amine sorbents such as monoethanolamine (MEA),diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropylamine(DIPA), or hydroxyethoxyethylamine (DGA). The liquid amine streamcontained the sorbed acid gas is typically regenerated by desorption ofthe sorbed gases in a separate tower with the regenerated amine and thedesorbed gases leaving the tower as separate streams. The various gaspurification processes which are available are described, for example,in Gas Purification, Fifth Ed., Kohl and Neilsen, Gulf PublishingCompany, 1997, ISBN-13: 978-0-88415-220-0.

The treatment of acid gas mixtures containing CO₂ and H₂S with aminesolutions typically results in the simultaneous removal of substantialamounts of both the CO₂ and H₂S. It is often desirable, however, totreat acid gas mixtures containing both CO₂ and H₂S so as to remove theH₂S selectively from the mixture, thereby minimizing removal of the CO₂.Selective removal of H₂S results in a relatively high H₂S /CO₂ ratio inthe separated acid gas which simplifies the conversion of H₂S toelemental sulfur using the Claus process. Selective H₂S removal isapplicable to a number of gas treating operations including treatment ofhydrocarbon gases from oil sands, coal and shale pyrolysis, refinery gasand natural gas having a low H₂S /CO₂ ratio and is particularlydesirable in the treatment of gases wherein the partial pressure of H₂Sis relatively low compared to that of CO₂ because the capacity of anamine to absorb H₂S from the latter type gases is very low. Examples ofgases with relatively low partial pressures of H₂S include syntheticgases made by coal gasification, sulfur plant tail gas and low-Joulefuel gases encountered in refineries where heavy residual oil is beingthermally converted to lower molecular weight liquids and gases.

Although primary and secondary amines such as MEA, DEA, DPA, and DGAabsorb both H₂S and CO₂ gas, they have not proven especiallysatisfactory for preferential absorption of H₂S to the exclusion of CO₂because in aqueous solution, the amines undergo more selective reactionwith CO₂ to form carbamates. The tertiary amine, MDEA, has been reportedto have a high degree of selectivity toward H₂S absorption over CO₂(Frazier and Kohl, Ind. and Eng. Chem., 42, 2288 (1950)), but itscommercial utility is limited because of its restricted capacity for H₂Sloading and its limited ability to reduce the CO₂ content of the gas.Similarly, diisopropylamine (DIPA) is relatively unique among secondaryamino alcohols in that its use has been reported, alone or with aphysical solvent such as sulfolane, for selective removal of H₂S fromgases containing H₂S and CO₂.

U.K. Patent Publication No. 2,017,524-A (Shell) disclosed that aqueoussolutions of dialkylmonoalkanolamines, and particularlyN,N-diethyl-monoethanolamine (DEAE), have higher selectivity andcapacity for H₂S removal at higher loading levels than MDEA solutions.Nevertheless, even DEAE is not very effective for the low H₂S loadingfrequently encountered in the industry. Also, DEAE has a boiling pointof 161° C., and as such, it is characterized as being a low-boiling,relatively highly volatile amino alcohol. Such high volatilities undermost gas scrubbing conditions result in large material losses withconsequent losses in economic advantages.

A number of severely sterically hindered amino ether compounds, notablyamino ether alcohols, diamino ethers and alkoxy amino ether alcoholshave been developed for the selective removal of H₂S in the presence ofCO₂. U.S. Pat. Nos. 4,405,581; 4,405,583; 4,405,585, 4,471,138 and4,894,178 and U.S. Patent Publication 2010/0037775 disclose these highlyeffective hindered amino ethers, their synthesis and use in selectivegas separation processes. Specific amino ethers described in thesepatents include BTEE (bis (tert.-butylamino-ethoxy)-ethane synthesizedfrom tertiary-butylamine and bis-(2-chloroethoxy)-ethane as well asEEETB (ethoxyethoxyethanol-tert-butylamine, synthesized fromtert-butylamine and chloroethoxy-ethoxyethanol). U.S. Pat. No. 4,894,178indicates that a mixture of BTEE and EEETB is particularly effective forthe selective separation of H₂S from CO₂. U.S. 2010/0037775 describesthe preparation of alkoxy-substituted etheramines as selective sorbentsfor separating H₂S from CO₂ Compared to aqueous MDEA, these severelysterically hindered amines lead to much higher selectivity at high H₂Sloadings

A significant problem arises with some of these absorbent materialsduring their transport from the manufacturing site to the location ofuse in cold climates; this problem arises when the pour point of thematerial is relatively high, typically at least −20° C. and the climaticconditions at their location of use is below that value or close to it.Such climatic zones include, for example, the North Sea areas of the UKand Norway, Ft. McMurray in Alberta, Canada and Billings, Mont. In zonessuch as these, there is the danger that the liquid will freeze solid orbecome unpourable to the extent that it cannot be readily orconveniently transferred or used unless they are thawed, but this takestime and provision needs be made for warmed defrosting and storagefacilities, especially on offshore platforms where space may be verylimited. It would therefore be desirable to transport the sorbentswithout having them solidify under the temperatures likely to beencountered during shipping from on location to another.

SUMMARY OF THE INVENTION

We have now found that liquid aminoether sorbents of high pour point canbe transported in cold climatic conditions without significant danger offreezing by the addition of water in judicious amounts. While both theaminoethes themselves and the water would freeze under those sameconditions, mixtures of the two are resistant to freezing. While otherpour point depressants could be expected to be effective also, the useof water is particularly attractive since the amines are typically usedin the form of an aqueous solution; shipping with the added watertherefore enables the use of additives which might interfere with theprocessing to be avoided. In addition, the use of water is economicaland avoids the use of possibly expensive chemicals. In offshorelocations, the addition of the water prior to shipping also reduces theamount of water needed to be added at the site of use offshore wherefresh water supplied may be limited.

According to the present invention, therefore, a liquid, severelysterically hindered, aminoether acid gas absorbent which is subject tofreezing in a cold climatic zone by reason of a pour point (ASTM D-97 orequivalent, e.g. Autopour), of −20° C. or higher, is renderedfreeze-resistant by mixing the aminoether with water prior; theaminoether/water mixture typically contains 10 to 80 more typically 10to 50, weight percent water, based on the weight of the aminoetheralthough the exact proportions can be adjusted according to theabsorbent itself and the projected temperatures during transport,storage and use. The amount of water necessary may be as low as 10 or 20percent for a useful lowering of the pour point. The aminoether/watermixture can transported through or into cold climatic zones with reducedrisk of freezing and can be stored there without being maintained at atemperature above the freezing point of the aminoether. The treatedaminoether absorbents in the form of the aqueous mixture can thereforebe transported from a first location to a second location in a coldclimatic zone for use in an acid gas treatment process with increasedconvenience.

In typical cases, the aminoether/water mixture will be transported froma first, relatively warmer climatic zone which has an ambienttemperature at which the aminoether remains unfrozen to a secondclimatic zone, colder relative to the first zone, which has an ambienttemperature below the freezing point of the aminoether itself; theconcentration of water in the aminoether/water mixture is adjusted to besufficient to depress the freezing point of the mixture to a temperaturebelow the ambient temperature of the second climatic zone. Theaminoether/water mixture can stored at a location in the second climaticzone at a temperature below the freezing point of the aminoether itself,e.g. in an unheated warehouse.

DETAILED DESCRIPTION Aminoether Absorbents

While the proposed transport scheme is applicable to the broad class ofliquid amines which may be used for the absorption of acidic gases suchas H₂S and CO2 from gas streams such a natural gas, syn gas etc, thepreferred amine sorbents are those which may be used for the selectivesorption of H₂S from acidic gas streams which are mixtures of H₂S withCO₂ and other acidic gases such as CS₂, HCN, COS and sulfur derivativesof C₁ to C₄ hydrocarbons. This preferred class of aminoethers isrepresented by the derivatives of diethylene glycol or polyethyleneglycols which contain severely sterically hindered amino groups as wellas by their corresponding derivatives derivatized on the alcohol groupto form the corresponding ether or ester derivatives and theircorresponding sulfonate and phosphonate salts. In general, the preferredseverely sterically hindered aminoether derivatives will have acumulative Es (Taft steric hindrance constant) value greater than 1.75(see below for further explanation of this constant and itscalculation).

Preferred examples of these aminoethers are disclosed in U.S. Pat. Nos.4,405,583; 4,405,585, 4,471,138, 4,894,178 and U.S. Patent Publication2010/0037775, to which reference is made for a full description of thesematerials, their synthesis and their use in selective acidic gasseparation processes. Their disclosures are summarized below forconvenience.

U.S. Pat. No. 4,405,583: The hindered diamino ethers disclosed in thispatent are defined by the formula:

where R¹ and R⁸ are each C₁ to C₈ alkyl and C₂ to C₈ hydroxyalkylgroups, R², R³, R⁴, R⁵, R⁶, and R⁷ are each hydrogen, C₁-C₄ alkyl andhydroxyalkyl groups, with certain provisos to define the adequatelyhindered molecule and m, n, and p are integers from 2 to 4 and o is zeroor an integer from 1 to 10. A typical diamino ether of this type is1,2-bis(tert-butylaminoethoxy) ethane, a diamino derivative oftriethylene glycol.

U.S. Pat. No. 4,405,585: The hindered amino ether alcohols disclosed inthis patent are defined by the formula:

where R¹ is C₁-C₈ primary alkyl and primary C₂-C₈ hydroxyalkyl, C₃-C₈branched chain alkyl and branched chain hydroxyalkyl and C₃-C₈cycloalkyl and hydroxycycloalkyl, R², R³, R⁴ and R⁵ are each hydrogen,C₁-C₄ alkyl and C₁-C₄ hydroxyalkyl radicals, with the proviso that whenR1 is a primary alkyl or hydroxyalkyl radical, both R² and R³ bonded tothe carbon atom directly bonded to the nitrogen atom are alkyl orhydroxyalkyl radicals and that when the carbon atom of R¹ directlybonded to the nitrogen atom is secondary at least one of R² or R³ bondedto the carbon atom directly bonded to the nitrogen atom is an alkyl orhydroxyalkyl radical, x and y are each positive integers from 2 to 4 andz is an integer from 1 to 4. Exempalry compounds of this type includethe amino ether alcohol tert-butylaminoethoxyethanol, a derivative ofdiethylene glycol.

U.S. Pat. No. 4,471,138: This patent discloses the desirability of usinga combination of a diamino ether with an aminoether alcohol. The twocompounds are represented by the respective formulae:

where x is an integer ranging from 2 to 6. This mixture can be preparedin the novel one-step synthesis, by the catalytic tertiarybutylamination of a polyalkenyl ether glycol,HO—(CH₂CH₂O)_(x)—CH₂CH₂—OH, or halo alkoxyalkanol. For example, amixture of bis-(tert-butylaminoethoxy)ethane (BTEE) andethoxyethoxyethanol-tert-butylamine (EEETB) can be obtained by thecatalytic tert-butylamination of triethylene glycol. The severelyhindered amine mixture, e.g., BTEE/EEETB, in aqueous solution can beused for the selective removal of H₂S in the presence of CO₂ and for theremoval of H₂S from gaseous streams in which H₂S is the only acidiccomponent, as is often the case in refineries.

U.S. Pat. No. 4,894,178: A specific combination of diamino ether andaminoalcohol represented by the respective formulae:

with x being an integer ranging from 2 to 6 and the weight ratio of thefirst amine to the second amine ranging from 0.23:1 to 2.3:1 andpreferably 0.43 to 2.3:1. This mixture can be prepared in the one-stepsynthesis, by the catalytic tert-butylamination of the correspondingpolyalkenyl ether glycol, for example, by the catalytictert-butylamination of triethylene glycol. This mixture is one of thepreferred absorbents for use in offshore gas processing.

US 2010/0037775: The reaction of a polyalkenyl ether glycol with ahindered amine such as tert-butylamine to form useful aminotherabsorbents is improved by the use of an alkoxy-capped glycol in order topreclude the formation of an unwanted cyclic by-product, tert-butylmorpholine (TBM). A preferred capped glycol is methoxy-triethyleneglycol although the ethoxy-, propoxy- and butoxy homologs may also beused. The reaction between triethylene glycol and tert-butylamine isshown to produce a mixture of bis-(tert-butylaminoethoxy) ethane andtert-butylaminoethoxyethoxyethanol in a weight ratio of about 65-67%:33% for a total yield of about 95% of the mixture over an extendedreaction time while the reaction with the alkoxy-capped glycol producesthe mono-amino reaction product in comparable yield after asignificantly shorter reaction time.

The aminoether compounds may be used in conjunction with other relatedmaterials such as an amine salt as described in U.S. Pat. No. 4,618,481.The severely sterically hindered amino compound can be a secondary aminoether alcohol or a disecondary amino ether. The amine salt can be thereaction product of the severely sterically hindered amino compound, atertiary amino compound such as a tertiary alkanolamine or atriethanolamine, with a strong acid, or a thermally decomposable salt ofa strong acid, i.e., ammonium salt or a component capable of forming astrong acid.

Similarly, U.S. Pat. No. 4,892,674 discloses a process for the selectiveremoval of H₂S from gaseous streams using an absorbent compositioncomprising a non-hindered amine and an additive of a severely-hinderedamine salt and/or a severely-hindered aminoacid. The amine salt is thereaction product of an alkaline severely hindered amino compound and astrong acid or a thermally decomposable salt of a strong acid, i.e.,ammonium salt.

A preferred class of aminoethers for offshore application is defined bythe formula:

R¹—N H—[CnH2n—O—]_(x)—OY

where R¹ is a secondary or tertiary alkyl group of 3 to 8 carbon atoms,preferably a tertiary group of 4 to 8 carbon atoms, Y is H or alkyl of 1to 6 carbon atoms, n is a positive integer from 3 to 8 and x is apositive integer from 3 to 6. The preferred R¹ group is tertiary butyland the most preferred amino ethers are those derived from triethyleneglycol (n is 2, x is 3). When Y is H, the amino ether is an amino etheralcohol such as tert-butylamino ethoxyethoxyethanol, derived fromtriethylene glycol; when Y is alkyl, preferably methyl, the amino etheris an alkoxy amino ether, with preference for tert-butylaminomethoxy-ethoxyethoxyethanol. The monoamino ethers may be used in blendswith diamino ethers in which the terminal OH group of the ether alcoholor the terminal alkoxy group of the alkoxy amino ether is replaced by afurther hindered amino group as expressed in the formula:

R¹—NH—[CnH₂n—O—]x-NHR²

where R¹, n and x are as defined above and R², which may the same ordifferent to R¹, is a secondary or tertiary alkyl group of 3 to 8 carbonatoms. A preferred diamino ether of this type is bis-(t-butylaminoethoxy) ethane which may conveniently be used as a mixture withtert-butylamino methoxy-ethoxyethoxyethanol in a weight ratio of about65-67wt%: 33-35wt%.

The severely sterically hindered secondary aminoether mentioned aboveare characterized by acyclic or cyclic moieties attached to the aminonitrogen atom(s). The term “severely sterically hindered” signifies thatthe nitrogen atom of the amino moiety is attached to one or more bulkycarbon groupings. Typically, the severely sterically hindered aminoetheralcohols have a degree of steric hindrance such that the cumulativeE_(s) value (Taft's steric hindrance constant) greater than 1.75 ascalculated from the values given for primary amines in Table V in D. F.DeTar, Journal of Organic Chemistry, 45, 5174 (1980), to which referenceis made for a description of this parameter.

Another means for determining whether a secondary amino compound is“severely sterically hindered” is by measuring its 15N nuclear magneticresonance (NMR) chemical shift. It has been found that the stericallyhindered secondary amino compounds have a 15N NMR chemical shift greaterthan about δ+40 ppm, when a 90% by wt. amine solution in 10% by wt. D₂Oat 35° C. is measured by a spectrometer using liquid (neat) ammonia at25° C. as a zero reference value. Under these conditions, the tertiaryamino compound used for comparison, methyldiethanolamine, has a measured15N NMR chemical shift value of δ27.4. For example,2-(2-tertiarybutylamino) propoxyethanol,3-(tertiarybutylamino)-1-propanol, 2-(2-isopropylamino)-propoxyethanoland tertiarybutylaminoethoxyethanol had measured 15N NMR chemical shiftvalues of δ+74.3, δ65.9, δ65.7 and δ+60. 5 ppm, respectively, whereasthe ordinary sterically hindered amine,secondary-butylaminoethoxyethanol and the non-sterically hindered amine,n-butylaminoethoxyethanol had measured 15N NMR chemical shift values ofδ+48.9 and 635.8 ppm, respectively. When the cumulative E_(s) values isplotted against the 15N NMR chemical shift values of the amino compoundsmentioned above, a straight line is observed. Amino compounds having an15N NMR chemical shift values greater than δ+50 ppm under these testconditions had a higher H₂S selectively than those amino compoundshaving an 15N NMR chemical shift less than δ+50 ppm.

The sterically hindered aminoether absorbents which require specialattention during cold storage, transport and use are those which have aPour Point (ASTM D-97 or equivalent) of −20° C. or higher. Absorbentswith lower pour points generally present no problem and accordingly itis not normally considered necessary for them to be mixed with waterbefore shipping out from the manufacturing location. Mixtures ofaminoether absorbents may require treatment with water if their freezepoints vary below those of the individual components of the mixtureshaving adequately low freeze/pour points.

Aminoether Blending, Transport, Storage

The aminoether absorbent or mixture of aminoether absorbents is blendedwith water to confer the desired resistance to freezing during shippingand storage. While the use of other suitable pour point depressantswould be similarly effective, the use of water is particularlyattractive since the amines are typically used in the form of an aqueoussolution; shipping with the added water therefore avoids the use ofother possibly expensive additives.

Exemplary freezing points for H₂S selective hindered aminoetherabsorbents are:

Pour Point Absorbent (° F./° C.) MEEETB (1) <−75/<−59 BisTEGTB (2)−60/−51 TEG, 7.3 wt %; −20/−29 TEGTB, 57.9 wt %; Bis-TEGTB, 34.9 wt %.EETB 95 wt % (3) −15/−26 Notes: 1. Methoxyethoxyethoxyethanol-t-butylamine (MEEETB) 97.8% purity, with methoxy-triethylene glycol (TEGM), 2.2 wt. % 2. Bis(t-butylamino) triethyleneglycol (BisTEGTB) 91.6 wt % purity, with triethylene glycol (TEG) 0.5 wt%; triethylene glycol-t-butylamine (TEGTB) 5.75 wt %. 3.Ethoxyethanol-t-butylamine.

While the freezing points of the BisTEGTB and MEEETB are low enough thatno problems are normally to be expected in the terrestrial environment,EETB and the blend of aminoethers have a pour/freezing point high enoughthat they can be expected to freeze in normal winter conditions in thehigher latitudes. By adding water, however, the freezing point can belowered to a useful extent, facilitating transport and storage inclimates with a harsh climate as shown by the following data recordingthe pour points achieved by adding water to these materials:

Concn. (wt %) Pour Point, Absorbent in water ° F./° C. TEG, 7.3 wt %;TEGTB, 57.9 wt %; 80 −30/−34 Bis-TEGTB, 34.9 wt %. TEG, 7.3 wt %; TEGTB,57.9 wt %; 60 −45/−43 Bis-TEGTB, 34.9 wt %. EETB 95 wt % 80 −30/−34 EETB95 wt % 60 −35/−37

The water, in an amount from about 10-50, preferably 20-40, weightpercent, based on the weight of the amine, is simply blended into theliquid aminoether using conventional mixing techniques, e.g., stirredtank mixer. The blended water/aminoether is then readied for shipping,for example, by loading into 200-I drums, ISO liquid containers, bulkliquid containers, Flexitanks, road tank trucks, rail tank cars, etc.The blended liquid is then shipped to the cold climatic location usingconventional means without the need for maintaining it at temperaturesabove freezing. Upon arrival at the cold climatic location, theaminoether/water blend can be diluted if necessary to the final desiredconcentration, typically from 5 to 30 v/v percent, and used in theabsorption process. Alternatively, it can be stored in an unheatedlocation until wanted for use.

The present transport scheme is useful when the aminoether is to beshipped through a climatic zone where sub-freezing temperatures for theaminoether, typically below about 0° C., prevail. It is especiallyuseful when shipping through a zone where the temperature is expected tobe more than ˜10 ° C. below the freezing temperature of the aminoether,especially when temperatures more than ˜20 ° C. below the freezing pointof the aminoether are expected.

The amount of water required to confer resistance to freezing willdepend upon the identity of the aminoether. As noted above, 10 to 45 wt.percent is normally sufficient with amounts within this range varyingaccording to the undiluted freezing point of the aminoether

1. A method for transporting a liquid aminoether acid gas absorbenthaving a pour point not lower than −20° F. for use in an acid gastreatment process through a cold climatic zone in which the aminoetherabsorbent is mixed with water prior to being transported through thecold climatic zone to form an aminoether/water mixture with about 10 to40 weight percent water, based on the weight of the aminoether.
 2. Amethod according to claim 1 in which the aminoether/water mixture istransported from a first climatic zone to a second climatic zone, thefirst climatic zone being warmer relative to the second climatic zone,the first climatic zone having an ambient temperature at which theaminoether remains unfrozen, the second climatic zone having an ambienttemperature below the freezing point of the aminoether itself, theconcentration of water in the aminoether/water mixture being sufficientto depress the freezing point of the mixture to a temperature below theambient temperature of the second climatic zone.
 3. A method accordingto claim 2 in which the aminoether/water mixture is stored at a locationin the second climatic zone at a temperature below the freezing point ofthe aminoether itself.
 4. A method according to claim 2 in which theaminoether comprises a diethylene glycol derivative or triethyleneglycol derivative containing a severely hindered amino group and havingan Es value (Taft) of at least 1.75.
 5. A method according to claim 2 inwhich the aminoether comprises tertiary-alkylamino diethylene glycol ora tertiary-alkylamino triethylene glycol.
 6. A method according to claim5 in which the aminoether comprises a mono-tertiary-alkylaminodiethylene glycol, a mono-tertiary-alkylamino triethylene glycol, abis-(tertiary-alkylamino) diethylene glycol or amono-tertiary-alkylamino triethylene glycol.
 7. A method according toclaim 6 in which the tertiary alkylamino group comprises atertiary-butylamino group.
 8. A method according to claim 6 in which theaminoether comprises a mixture of bis-(tertiarybutylaminoethoxy)-ethane(BTEE) and ethoxyethoxyethanol-tertiary-butylamine (EEETB).
 9. A methodaccording to claim 6 in which the aminoether comprises a mixture inwhich the weight ratio of the diamino compound to the monoamino in themixture is from 0.23:1 to 2.3:1.
 10. A method according to claim 2 inwhich the aminoether comprises a alkoxy-tert-alkylamino-ether.
 11. Amethod for storing a liquid aminoether acid gas absorbent for use in anacid gas treatment process in a cold climatic zone having an ambienttemperature below the freezing point of the aminoether itself whichcomprises mixing the aminoether absorbent with water prior to storage inthe cold climatic zone to form an aminoether/water mixture with about 10to 45 weight percent water, based on the weight of the aminoether.
 12. Amethod according to claim 11 in which the aminoether/water mixture istransported from a first, relatively warmer climatic zone to the coldclimatic zone, the first climatic zone having an ambient temperature atwhich the aminoether remains unfrozen, the concentration of water in theaminoether/water mixture being sufficient to depress the freezing pointof the mixture to a temperature below the ambient temperature of thecold climatic zone.
 13. A method according to claim 12 in which theaminoether/water mixture is transported from the first climatic zone tothe location in the cold climatic zone at a temperature below thefreezing point of the aminoether itself.
 14. A method according to claim11 in which the aminoether comprises an ethylene glycol or polyethyleneglycol compound containing a severely hindered amino group having an Esvalue (Taft) of at least 1.75.
 15. A method according to claim 11 inwhich the aminoether comprises tertiary-alkylamino diethylene glycol ora tertiary-alkylamino triethylene glycol.
 16. A method according toclaim 15 in which the aminoether comprises a mono-tertiary-alkylaminodiethylene glycol, a mono-tertiary-alkylamino triethylene glycol, adi-tertiary-alkylamino diethylene glycol or a di -tertiary-alkylaminotriethylene glycol.
 17. A method according to claim 16 in which thetertiary alkylamino group comprises a tertiary-butylamino group.
 18. Amethod according to claim 16 in which the aminoether comprises a mixtureof bis-(tertiarybutylaminoethoxy)-ethane (BTEE) andethoxyethoxyethanol-tertiarybutylamine (EEETB).
 19. A method accordingto claim 16 in which the aminoether comprises a mixture in which theweight ratio of the diamino compound to the monoamino is from 0.23:1 to2.3:1.
 20. A method according to claim 2 in which the aminoethercomprises an alkoxy-tert-alkylamino-ether.